As colorants for printing inks, water-insoluble colorants excellent in fastness such as waterproofness and light fastness, for example, pigments have been used widely to date. To use a water-insoluble colorant as a colorant for a water-based ink, it is required to stably disperse the water-insoluble colorant in an aqueous medium. Water-based inks of the colorant dispersion type are hence used, each of which contains a water-insoluble colorant evenly dispersed in an aqueous medium by the addition of a dispersant such as a high-molecular compound or surfactant.
In recent years, water-based inks of such colorant dispersion type have also been used in inkjet recording from the standpoint of image fastness. In the inkjet recording, it is attempted to provide colorant dispersion particles, which are dispersed in an ink, with agglomerating property and water-insolubility so that the ink can exhibit improved fixing property and waterproofness on paper. The provision of such properties to the colorant particles, however, leads to a reduction in the dispersion stability of the colorant particles in the ink, thereby developing potential problems such that the colorant particles may easily agglomerate and settle during storage of the ink; the resulting images tend to exhibit uneven print density; and clogging attributable to drying of the ink tends to occur at nozzle tips of an inkjet recording system to result in reduced ejection stability.
In an attempt to solve the above-mentioned problems, an ink which contains water-insoluble colorant having a specific structure is proposed in the bellow Patent Reference 1. In this ink, however, only water-insoluble property of the colorant as a colorant has been considered; the ink has some effect on an imaging property formed with the ink, but it has great problems in its long-term storage stability and its ejection stability.
An ink also is proposed in the bellow Patent Reference 2, which ink contains a high-molecular dispersant having a specific structure in a hydrophobic segment in its high-molecular structure. In this high-molecular dispersant, its hydrophilic segments closely associated with the compatibility with ink medium are not taken into account; consequently the ink has such problems that the dispersion stability of the colorant particles are significantly lowered during its storage with long-term or high-temperature. Because of such problems, specifically the dispersion stability of the ink are significantly reduced when an ink composition are extremely changed in response to concentration of the ink at nozzle tips of a recording head, which concentration inevitably are caused on use of an ink in an inkjet system; the lowered dispersion stability of an ink, thereby, leads the reduced ejection stability of the ink. Further the ink has such problems that, upon use of the ink in an ink jet recording system applying thermal energy to the ink to cause the ink to fly onto a recording medium, the colorant particles agglomerate extremely because of the generation of heat; the agglomeration makes ejection of the ink impossible.
Patent Reference 1: JP 1998-120956 A
Patent Reference 2: JP 2002-332440 A